With remarkable progress of lasers in recent years, a high output and compact solid laser or semiconductor laser having a light emission region in the range from near infrared to infrared becomes easily available. These lasers are very useful as a light source for exposure in producing a printing plate directly from digital data of a computer or the like.
The positive working lithographic printing plate material for use with an infrared laser contains, as essential components, an alkali aqueous solution-soluble binder resin and an IR dye or the like capable of absorbing light to generate heat, where the exposed area (non-image area) dissolves in an alkali developer to form a lithographic printing plate.
However, such a positive working lithographic printing plate material for use with an infrared laser has a problem in that a sufficiently large difference is not attained between the dissolution resistance of the unexposed area (image area) against developer and the solubility of the exposed area (non-image area) under various use conditions, therefore, the sensitivity is low and over development or development failure is liable to occur due to the change of use conditions. Furthermore, the surface state readily fluctuates, for example, fine scratches are generated even by the touching with the surface at the handling, and such fine scratch or slight change on the surface causes increase of the solubility, as a result, the unexposed area (image area) dissolves at the development to leave scratch trace and this gives rise to the deterioration of impression capability or inking failure.